With increased emphasis being placed upon energy conservation in recent years, it has become important that air conditioning systems designed for space heating and cooling be operated in the most efficient and economical manner possible. In response to this desired objective, heating and cooling systems have been redesigned in order to improve their efficiencies, increased emphasis has been placed upon the use of more energy-efficient types of heating and cooling systems; work has been done on control systems which enable such systems to operate at the minimum level required in order to maintain a desired level of comfort conditioning; and building occupants have been asked simply to tolerate higher temperatures during the cooling season and lower temperatures during heating. As examples of the foregoing, one may note the increased use of heat pumps of the vapor compression type for supplying space heating, the use of solar heating to provide at least a portion of a building's space heating requirements, when possible; and the increased use of an economizer cycle wherein outdoor ambient air is used to satisfy building cooling loads during those times that the condition of the outdoor air is suitable for this purpose.
A control concept which has seen increasing popularity recently is referred to commonly as "temperature setback" wherein the comfort condition, usually temperature, to be maintained within a conditioned zone is varied from a normal level to an economy level during those times that it is feasible to do so. For example, during the heating season, the temperature within a particular conditioned zone may be maintained at say 70.degree. F. during the daytime hours when the zone is occupied, and at, say 60.degree. F. during the nighttime hours when the zone is either unoccupied or its occupants are asleep. In the case of commercial structures, it is often possible that the temperature may be set back over a weekend period when the zone is unoccupied. Similarly, during the cooling season, the temperature may be maintained at a normal level of say, 72.degree. F. when the zone is occupied, at at an economy level of, say, 80.degree. F. when it is unoccupied.
While there is no doubt that temperature setback is an effective method of reducing energy consumption for air conditioning systems wherein a single type of heating and/or cooling apparatus is involved, questions have been raised as to the applicability of the temperature setback concept in conjunction with systems of the type wherein two sources of either heating and/or cooling are available, one of which exhibits more favorable energy consumption characteristics than the other. A specific example of such a system would be one in which an electrically operated heat pump of the vapor compression type, representing a preferred conditioning means, and electrical resistance-type electrical heating means as a back-up conditioning means are provided, the former exhibiting more favorable energy consumption characteristics than the latter.
Similar considerations relative to the applicability of the temperature setback concept would be presented in the case of a cooling system wherein both a refrigeration system and an economizer cycle are provided, the latter of which would exhibit more favorable energy consumption characteristics than the former in providing cooling of a conditioned zone.
The present invention addresses the problems presented in adapting temperature setback to air conditioning systems of this type and sets forth control schemes therefor which insure that energy savings will be realized.